Positional Tuning of Photophysics and Catalysis in Methoxy-Substituted Heteroleptic Copper(I) Complexes

A series of heteroleptic copper(I) photosensitizers based on methoxy-substituted 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline ligands was synthesized to investigate the influence of substitution patterns on structure and function. Methoxy groups were introduced in <i>ortho</i>-, <i>meta</i>-, and <i>para</i>-positions of the phenyl rings. Single-crystal X-ray diffraction and DFT calculations confirmed the expected tetrahedral geometry with position-dependent aryl torsion. Photophysical studies reveal that <i>ortho</i>/<i>para</i>-substitution enhances absorptivity, emission quantum yields, and excited-state lifetimes compared to the <i>meta</i>/unsubstituted complexes. The <i>ortho</i>-substituted complex shows the strongest electron-donating effect, reflected in the most cathodic ligand reduction <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>(</mml:mo><mml:msubsup><mml:mrow><mml:mi>E</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow><mml:mrow><mml:mo>/</mml:mo></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:mrow><mml:mrow><mml:mi>red</mml:mi></mml:mrow></mml:msubsup><mml:mo>=</mml:mo><mml:mo>-</mml:mo><mml:mn>2.11</mml:mn><mml:mspace></mml:mspace><mml:mi>V</mml:mi><mml:mo>)</mml:mo></mml:math> and the least oxidizing excited state potential (<i>E</i>* = 0.46 V). Temperature-dependent luminescence and emission lifetimes are consistent with thermally activated delayed fluorescence (TADF) across the series and reveal substitution-controlled singlet-triplet energy gaps Δ<i>E</i>_ST. Complemented by step-scan FTIR studies, the predominant excited state was identified and analyzed, highlighting the impact of spin density location on both energy- and electron-transfer reactivity. The photocatalytic relevance was demonstrated in three benchmark reactions: singlet oxygen generation (energy transfer, demonstrated by the photooxidation of diphenylfuran to <i>cis</i>-dibenzoylethylene), hydrogen evolution from water, and reductive dehalogenation of aryl halides (electron transfer). In hydrogen evolution, the <i>para</i> isomer gave the highest initial rate and a TON of 590 at 20 h, while the <i>ortho</i> isomer remained active up to 36 h with a TON of 530. Stern-Volmer quenching in THF with TEA confirms a reductive pathway under these conditions. In the photocatalytic dehalogenation, activity trends were substrate-dependent, reflecting a balance between excited-state driving force (<i>E</i>*) and ground-state reducing power <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>(</mml:mo><mml:msubsup><mml:mrow><mml:mi>E</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow><mml:mrow><mml:mo>/</mml:mo></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:mrow><mml:mrow><mml:mi>red</mml:mi></mml:mrow></mml:msubsup><mml:mo>)</mml:mo></mml:math>. Together, these results establish clear position-property-performance relationships to guide Cu(I) photosensitizer design.